An electronic device such as a laser printer has a paper feed roller (i.e., driven unit) coupled with a motor and disposed inside a main unit casing, and this motor drives the paper feed roller to rotate and deliver paper to a predetermined section.
A brushless DC motor is the type generally used for the above motor, which comprises a stator having a plurality of magnetic poles disposed at first predetermined intervals along an outer periphery thereof, and a rotor disposed around the outer periphery of the stator. The rotor has a structure comprising permanent magnets disposed to an inner periphery thereof with their poles magnetized alternately to have different polarities at second predetermined intervals.
The stator is also provided with an extended portion formed on each of the magnetic poles in a manner to extend from the base of the magnetic pole into a direction generally parallel to the permanent magnets for improvement of a driving efficiency.
That is, the permanent magnets are made larger in width (i.e., the width in a direction orthogonal to the circumferential direction) than a width at the base of the magnetic poles of the stator in the same direction in order to bring the permanent magnets as close as possible to a magnetic sensing element for magnetically sensing rotation of the rotor. Extended portions are so formed as to extend from bases of the magnetic poles into the direction generally parallel to the permanent magnets to increase an area where the magnetic poles of the stator confront the permanent magnets and to gain a high driving force and efficiency. Patent literature 1, for instance, discloses a technique similar to the above.
In the conventional motor, the extended portions formed to extend from the bases of the magnetic poles into the direction parallel to the permanent magnets in the manner as described above can increase the area where the permanent magnets of the rotor confront the magnetic poles of the stator. It has been believed in general that the above structure can increase the driving force, and hence improve the driving efficiency.
According to the studies made by the inventors of this application, however, it was found not necessarily possible to increase the driving force by simply providing the extended portions.
In other words, the extended portions extended from the electrodes of the stator are enlarged as much as possible since it is suggested according to the general conception that the driving force can be increased by enlarging the area where the permanent magnets of the rotor confront the magnetic poles of the stator. When the extended portions are enlarged, however, an amount of magnetic flux from the confronting permanent magnets also increases proportionally. This gives rise to a problem that magnetic saturation can result in magnetic circuits of the stator communicating with the magnetic poles, and it interferes with improvement of the driving force and the driving efficiency.
In addition, there occurs an eddy-current loss in the extended portions since the magnetic flux from the magnets intersects orthogonally with the extended portions. The eddy-current loss occurs due to an eddy current generated when the magnetic flux intersects a conductor so that the larger the area where the magnetic flux intersects orthogonally the more the eddy-current loss is generated. It thus causes a problem of preventing improvement of the driving force and the driving efficiency.    Patent Literature 1: Japanese Patent Unexamined Publication, No. 1997-285044